The present invention relates to an amplifier circuit and, more particularly, to an amplifier circuit of the type having a capacitor which is inserted on the input side of the circuit in order to adjust the direct current (DC) operating point in relation to preceding circuitry.
If an amplifier circuit is preceded by an external circuit, a capacitor is usually installed in the amplifier circuit so as to adjust the DC operating points of the external circuit and of the amplifier circuit. A bias voltage to be applied to such an amplifier circuit may be set up by any of two different approaches as well known in the art, i.e., feeding back the output of the amplifier circuit via a resistor, and dividing the power source voltage by the resistor. A problem with a circuit adapted to set up a bias voltage as described is that when a power source of the external circuit is operated intermittently to provide a battery saving feature, the enabling of the amplifier circuit is delayed by a certain time constant which is determined by the bias setting resistor and by the capacitor on the input side.
The delay in the enabling of the amplifier circuit often becomes a serious problem from the system standpoint. For example, when an amplifier circuit with any of the bias setting arrangements as described above is installed in a phase locked loop (PLL) which is operated in an intermittent or battery-saving mode, the so-called lock-in time becomes impractically long.